The present invention relates to a web breakage detector for web processing machines and more particularly for web feed printing machines, comprising a device for monitoring the web tension and adapted to produce a signal in the event of the web tension decreasing.
A device of this type is described in German patent publication 3,939,226 A1. In the case of this known arrangement the monitoring device consists of a photoelectric detector whose optical path is intercepted by one longitudinal edge of the web and a blowing nozzle acting on the web with a force exceeded by the normal web tension. In this case it is necessary to set the photoelectric detector and the nozzle associated with it exactly at the edge of the web. A further point to be considered is that the force of the blown air from the nozzle has to be set in accordance with the quality of the paper. Accordingly there is a generally elaborate adjustment routine; and furthermore there is the danger of failure to perform the correct adjustments. The known arrangement is consequently not sufficiently easy to operate and not sufficiently reliable.